Asymmetric pathogen spillover favors exotic plants over natives

Exotic plants can escape from specialist pathogenic microorganisms in their new range, but may simultaneously accumulate generalist pathogens. This creates the potential for pathogen spillover, which could alter plant-competitive hierarchies via apparent competition. To assess the potential for and consequences of pathogen spillover in invaded communities, we conducted a community-level plant-soil feedback experiment in experimental communities that ranged in the extent of exotic dominance, using next-generation sequencing to characterize sharing of putatively-pathogenic, root-associated fungi (hereafter, ‘pathogens’). Exotic plants outperformed natives in communities, despite being subject to stronger negative plant-soil feedbacks in monoculture and harboring higher relative abundance of pathogens. Exotic plants made more general associations with pathogens, making them more prone to sharing pathogens with natives and exerting apparent competition. These data suggest that exotic plants accumulate generalist pathogens that are shared with native plants, conferring an indirect benefit to exotic, over native plants.

Parental feeding preferences rather than sibling competition determine the death of smaller nestlings in asynchronous broods

Hatching asynchrony is a reproductive tactic that, through the creation of competitive hierarchies among offspring, allows parents for a quick adjustment of brood size via the death of smaller nestlings. This strategy is considered to be adaptive in case of unpredictable and/or poor environments in which it would guarantee that at least larger nestlings will fledge. Brood reduction is the usual outcome in asynchronously hatched broods since first-hatched nestlings are larger and get a disproportionately larger share of the food delivered by parents, often leading the youngest nestling to starve to death soon after hatching. However, we still do not know the proximate mechanisms of such brood reduction. One possibility is that the smallest nestling is not fed because larger nestlings outcompete it, which implies that nestlings control resource allocation. Alternatively, parents might actively ignore the persistent begging from their smallest nestling, which would involve that parents control food allocation. To determine whether parents or nestlings ultimately induce brood reduction in this situation, we experimentally created asynchronous broods of Eurasian blackbird (Turdus merula) nestlings and quantified food allocation by parents in two different situations: when sibling competition was allowed and, alternatively, when competition was prevented by physically separating nestlings within the nests by using wooden barriers. Our results showed that experimentally introduced smaller nestlings received less food than their larger nestmates both when competition among nestlings was allowed and when it was prevented. When adult males and females are considered separately, males fed the smallest nestling less often regardless of whether sibling competition was allowed or not, but adult females showed no differences. We can conclude that the smallest nestling starves mainly because parents actively ignore its begging. The higher competitive ability of the larger nestlings seem to have little effect given that although the smallest nestling is fed at a higher rate when physical interactions are prevented by the wooden barrier than when not, this difference is not significant. These findings suggest that parents rather than nestlings have the main control over food allocation.

Competitive hierarchies among three species of juvenile coral reef fishes

Interspecific competition is often asymmetric, and it can limit the spatial distributions of competitively inferior species within a community. When asymmetric competition involves 2 or more component species, the ranking of species' competitive abilities may form competitive hierarchies (all species of higher rank out-compete all species of lower rank) or competitive networks (at least 1 species of lower rank out-competes =1 species of higher rank). Expectations of resource monopolization and patterns of distribution and abundance among species in competitive networks are expected to differ from those in competitive hierarchies. We conducted a field experiment to evaluate the relative competitive abilities of juveniles of 3 closely related species of reef fish (bird wrasse Gomphosus varius, fivestripe wrasse Thalassoma quinquevittatum and the sixbar wrasse T. hardwicke) on Moorea, French Polynesia. We controlled for intrinsic variation in survivorship among species and found that competition among these 3 species was highly asymmetric, resulting in a simple competitive hierarchy (sequence of competitive ability from superior to inferior competitors): fivestripe wrasse > bird wrasse > sixbar wrasse. We surveyed densities of the 3 reef fish species on 55 patch reefs and observed significant negative spatial covariation between superior and inferior competitors, consistent with competitive hierarchies that limit the spatial distributions of the inferior species (sixbar wrasse). Our work demonstrates that intense asymmetric competition and the formation of competitive hierarchies may be an important determinant of resource monopolization and patterns of distribution and abundance in reef fishes. © Inter-Research 2013.

Competitive hierarchies among three species of juvenile coral reef fishes

Interspecific competition is often asymmetric, and it can limit the spatial distributions of competitively inferior species within a community. When asymmetric competition involves 2 or more component species, the ranking of species' competitive abilities may form competitive hierarchies (all species of higher rank out-compete all species of lower rank) or competitive networks (at least 1 species of lower rank out-competes =1 species of higher rank). Expectations of resource monopolization and patterns of distribution and abundance among species in competitive networks are expected to differ from those in competitive hierarchies. We conducted a field experiment to evaluate the relative competitive abilities of juveniles of 3 closely related species of reef fish (bird wrasse Gomphosus varius, fivestripe wrasse Thalassoma quinquevittatum and the sixbar wrasse T. hardwicke) on Moorea, French Polynesia. We controlled for intrinsic variation in survivorship among species and found that competition among these 3 species was highly asymmetric, resulting in a simple competitive hierarchy (sequence of competitive ability from superior to inferior competitors): fivestripe wrasse > bird wrasse > sixbar wrasse. We surveyed densities of the 3 reef fish species on 55 patch reefs and observed significant negative spatial covariation between superior and inferior competitors, consistent with competitive hierarchies that limit the spatial distributions of the inferior species (sixbar wrasse). Our work demonstrates that intense asymmetric competition and the formation of competitive hierarchies may be an important determinant of resource monopolization and patterns of distribution and abundance in reef fishes. © Inter-Research 2013.

Competitive hierarchies, antibiosis, and the distribution of bacterial life history traits in a microbiome*

Microbiome manipulation requires an understanding of how species interact within communities. Can outcomes of ecological interactions be predicted from microbial life history traits, the identity of the species, or both? We addressed these questions by studying the competitive interaction network in a community of 40 endophytic Pseudomonas spp. bacterial isolates from a native plant. Pairwise competition experiments revealed competitive dominance of P. fluorescens over P. syringae strains within this microbiome-derived community. P. syringae strains with higher growth rates won more contests, while P. fluorescens strains with shorter lag times and lower growth rates won more contests. Adding to their competitive dominance, P. fluorescens strains often produced antibiotics to which few P. syringae strains were resistant. Many competitive outcomes among P. syringae strains were predicted to be reversed by P. fluorescens inhibitors because indirect benefits accrued to less competitive strains. P. fluorescens strains frequently changed competitive outcomes, suggesting a critical role of strains within this bacterial clade in structuring plant microbiome communities. Microbial traits also may provide a handle for directing the outcome of colonization processes within microbiomes.

Habitat heterogeneity mediates effects of individual variation on spatial species coexistence

Numerous studies have documented the importance of individual variation (IV) in determining the outcome of competition between species. However, little is known about how the interplay between IV and habitat heterogeneity (i.e. variation and spatial autocorrelation in habitat quality) affects species coexistence at the landscape scale. Here, we incorporate habitat heterogeneity into a competition model with IV, in order to explore the mechanism of spatial species coexistence. We find that individual-level variation and habitat heterogeneity interact to promote species coexistence, more obviously at lower dispersal rates. This is in stark contrast to early non-spatial models, which predicted that IV reinforces competitive hierarchies and therefore speeds up species exclusion. In essence, increasing variation in patch quality and/or spatial habitat autocorrelation moderates differences in the competitive ability of species, thereby allowing species to coexist both locally and globally. Overall, our theoretical study offers a mechanistic explanation for emerging empirical evidence that both habitat heterogeneity and IV promote species coexistence and therefore biodiversity maintenance.

An Empiricist’s Guide to Modern Coexistence Theory for Competitive Communities

While most ecological theories have historically invoked niche differences as the primary mechanism allowing species coexistence, we now know that species coexistence in competitive communities actually depends on the balance of two opposing forces: niche differences (ND) that determine how species limit their own growth rate versus that of their competitor, and relative fitness differences (RFD) that establish competitive hierarchies among species. Several different empirical methods have been proposed for measuring ND and RFD in order to make predictions about coexistence of species, yet it remains unclear which method(s) are appropriate for a given empirical study and whether or not those methods actually yield the same information. Here we summarize and compare five different empirical methods, with the aim of providing a practical guide for empiricists who want to predict coexistence among species. These include two phenomenological methods that estimate ND and RFD based on observing competitive interactions among species; two mechanistic methods that estimate ND and RFD based solely on information about species’ resource requirements; and a fifth method that does not yield ND and RFD but describes the impacts of those forces within communities. Based on the specific requirements, limitations, and assumptions of each approach, we offer a series of decision steps that can be used to determine which method(s) are best for a given study system. In particular, we show there are important tradeoffs between mechanistic methods, which require detailed understanding of species niches and physiology but are more tractable experimentally, and phenomenological methods which do not require this detailed information but can be impractical for some study designs. Importantly, we show that although each method can be used to estimate ND and RFD, the methods do not always yield the same values. Therefore we caution against future syntheses that compile these estimates from different empirical studies. Finally, we highlight several areas where modern coexistence theory could benefit from additional empirical work.